There are many coal deposits around the world that contain a considerable amount of uranium and other valuable minerals. In the United States uranium entrained in coal was known as early as 1874 and was so reported by Berthoud, "On the occurrence of uranium etc. in the tertiary formation of Colorado Territory," Philadelphia Academy of Natural Sciences Proceedings, 1875. The intense interest in uranium spawned by World War II led to numerous discoveries of uranium in coal in North Dakota, South Dakota, New Mexico, Texas and Wyoming.
One such discovery is reported in detail in U.S. Geological Survey Bulletin 1099-B, "Uranium-Bearing Coal in the Eastern Part of the Red Desert Area, Wyoming 1962." In this deposit one seam of coal containing 2 million tons has an average uranium content of 0.01%. Localized concentrations of coal has up to 0.14% uranium values in the ash.
In several cases in the United States where the uranium-bearing coal was overlain by shallow overburden, the coal was strip mined and then burned to ash. The ash provided concentrations of uranium that facilitated commercial recovery. Since the fuel value of the coal was not utilized, such procedures were limited by economics to coal deposits near the surface.
Generally, uranium bearing coals tend to be of poor quality as fuels due to relatively high ash and moisture contents. For the most part the uranium values of interest appear to be emplaced as a secondary deposition from uranium-bearing waters percolating through the coal, and such deposits are normally associated with a porous rock formation overlying the coal. In a setting of this type uranium values reach their maximum in the upper portion of the coal seam. In some cases the upper portion of the coal seam is carbonaceous material that is very effective in removing uranium values from percolating water, but is very poor as a fuel. Such deposition of uranium values by adsorption by the coal is an irreversible process.
It is well known in the art how to produce coal in situ. Methods of such production are taught in U.S. Pat. Nos. 3,924,680, 3,948,320, 3,952,802, 3,987,852, 4,010,800, 4,010,801 and 4,018,481. One problem in producing coal in situ by burning is the tendency toward flame override, resulting in rapid consumption of the uppermost coal in the seam with corresponding difficulties in consuming the coal in the lower part of the seam. Flame override is an advantage when it is desirable to convert the uppermost portion of the coal to ash. Further, in situ burning processes can be accomplished at depths unfavorable for strip mining or for conventional underground mining. In many cases the removal of the uppermost portion of the coal has little effect on the value of the coal deposit as a fuel because of the low quality of the coal consumed.
It is well known in the art how to extract uranium values from porous host material by leaching with a liquid that will take the uranium values into solution. Common leaching liquids for this purpose include solutions of sodium carbonate and solutions of sulfuric acid. Uranium values taken into solution are readily extracted by one of several commercial processes.
No particular novelty is claimed in producing coal in situ. No particular novelty is claimed in extracting uranium values by leaching. It is an object of the instant invention to teach methods combining those used in producing coal in situ together with those used in leaching uranium values in order to recover uranium and other minerals from coal in situ. Those skilled in the art will be able to envision modifications to methods taught herein that can be used to remove other valuable constituents of the residual ash such as vanadium, chromium, germanium, nickel, and the like.
Other objects, capabilities and advantages of the instant invention will become apparent as the description proceeds and in conjunction with the drawings.